1. Field of the Invention
This invention relates to an audio output amplifier having a stand-by circuit, and more particularly to a protection circuit for preventing output elements from being damaged at the time of short circuit of the output terminal.
2. Description of the Related Art
In the prior art, an audio output amplifier includes a stand-by circuit for determining whether the operation condition is a stand-by mode or normal operation mode in response to a stand-by signal, a bias circuit controlled in response to an output signal of the stand-by circuit to generate a predetermined bias voltage, a protection circuit controlled by an output signal of the stand-by circuit, an amplifier circuit connected to receive an audio signal and a bias voltage from the bias circuit and amplify the audio signal, and an output stage element (transistor) driven by an output signal from the amplifier circuit. In the audio output amplifier with the construction as described above, a non-stand-by mode (operative mode) or stand-by mode is selectively set according to the stand-by signal supplied to the stand-by circuit. Transition from the operative mode to the stand-by mode is rapidly effected. This causes a voltage at an output terminal connected to the output stage element to be rapidly changed, thereby causing an impulsive sound to be generated from a load such as a speaker connected to the output terminal. The stand-by circuit is also used to attain the audio output muting function, for example.
In order to prevent the impulsive sound from being generated, it is a common practice to delay the stand-by signal supplied to the stand-by circuit by using a CR time constant circuit. When the stand-by signal is delayed, the bias voltage from the bias circuit slowly drops, and therefore the output voltage generated from the output terminal is slowly lowered. This successfully prevents occurrence of the impulsive sound.
In a case where an abnormal condition occurs, for example, when the output terminal of the audio output amplifier is grounded, the protection circuit is operated. If transition from the operative mode to the stand-by mode occurs in the abnormal condition, the output signal of the bias circuit is slowly lowered in the transition process. At this time, since the protection circuit is immediately set into the nonoperative condition, a bias voltage is supplied to the bias circuit without any protection. Therefore, in this case, the amplifier circuit amplifies the audio signal based on the bias voltage. For this reason, if the abnormal condition as described above occurs in the transition process from the operative mode to the stand-by mode, the output stage element may be damaged.